iPhone 7 Plus Restarting after screen replacement and stuck in 1%

Hello,

I m going to try to explain everything as best as i can, this is the thing:

I have an iPhone 7 Plus that had a cracked screen and the battery was pretty bad (50% condition) so i bought an iPhone 7 Plus that had activation lock and decided to just swap my board to the locked one because it was in better condition (screen and housing) and then the problems started, the first thing i noticed was the battery charge was stuck in 1% and sometimes showed 100% and after 5-6 minutes the phone restarted, i restored the phone with itunes thinking that would fix the problem but it didnt, then i swapped the boards again to the old housing but it was still the same, i even tried with the old battery and still the same (restarting every 5 minutes and stuck in 1%)

So i took the phone to a repair shop and they told me that the phone had a bad Charge IC and they replaced it but it didnt solve the problem, i dont know to much about microsoldering or any of that stuff so i told them to keep looking for the problem, they had it for 2 weeks and told me they replaced almost every transitor, filter and even tried a few charge ICs but is still the same so they gave up and returned the phone to me as it was.

Now that i have the phone in hands i tried charging the battery with the locked board i have and then put it back in to the defective one and it shows higher battery % but it give some erratic numbers (like sometimes is 56% and jumps to 65%) also on the battery condition it says service and keep restarting

Can someone give an idea of what is happening with this iPhone or should i just give up.